COMMUNITY FORUM

furnace blower won't always shut off, heating mode only

Hi! I recently bought a house with a Dettson oil-fired forced air furnace, with a Heil heat pump. Both were installed in 1989, according to the real estate agent's listing. I had no problem during the cooling season, but as soon as I switched the thermostat (Honeywell mechanical thermostat) to the heat mode, sometimes the blower will stay on forever. I checked every wire, and they are all OK. Even with the thermostat off the wall, the blower is still working!!! If I switch off the power to the furnace, and switch it back on, the blower stops, until the next time it is called to blow by the thermostat contacts. If I switch the thermostat to "emergency heat" which calls for the oil burner instead of the heat pump, I don't have this problem. Could someone help me address this issue? Thanks.

I noticed that while the furnace blower stays on, the heat pump does not work. I can shut the heat pump's breaker off, the furnace blower will still blow. And if it is the fan relay, is it possible that it sticks in the "on" position only when I use the heat pump for heat, not for cooling, and it also works fine when I use the furnace's oil burner for heat? Also, I found by looking at the bills I got when I bought the house, the furnace was retrofitted after the heat pump was installed on the old furnace. Thanks for your help.